Taking
a Grand Virtual Voyage in the Milky Way

Leaving Uranus,
we now move deeply into the outer Solar System and encounter that last gas giant
planet, Neptune. With a gassy atmosphere full of methane, and (we believe) a
deep ocean underneath, Neptune also has the swiftest winds of any planet in
the Solar System...

We think Triton
may look quite a bit like Pluto, the most remote planet in the Solar System,
but we have never been there. Even the Hubble Space Telescope is incapable of
showing us much of tiny Pluto's surface. It is that far away and that small,
measuring smaller than seven planetary moons!

Our journey continues
as we leave the comfort of the Solar System, picking up speed and changing direction
to intercept the Sun's closest stellar neighbors, the triple-star, Alpha Centauri
system. Visible from the southern hemisphere back on Earth, the system rushes
past.

We still have
a long distance to cover before we leave the Milky Way Galaxy -- tens of thousands
of lightyears, as a matter of fact -- so let's stop by some interesting objects
on the way.

Here we come to
the Pleiades star cluster, a grouping of a couple of hundred stars, all relatively
young. The Pleiades move through the Galaxy together, and here we see them passing
through a cloud of interstellar gas and dust; note how their light is scattered
by particles in the cloud, an example of nebulosity.

Since "nebulae"
are my favorite objects, we'll stay here a minute. The Pleiades cluster contains
an example of a reflection nebula; there are other kinds of nebulae.

The
planetary nebula NGC 3132. Image courtesy of AURA/STScI and NASA.

Planetary nebulae
are made of small shells of gas thrown off by dying, low-mass stars. The Sun
will produce a planetary nebula as it dies in about six billion years. Here
we see the Helix nebula, a good example of a planetary nebula; note the small
bluish star at the very center of the nebula -- the dead star's core!

Emission nebulae
are composed of gases (mostly hydrogen) that have been excited by stars' emitted
radiation. When gas atoms get excited, they glow like a neon sign. The Great
Nebula in Orion is 1500 lightyears away, but the young stars inside the Nebula
excite the gas so much that the Orion Nebula is bright even to the naked eye.

Finally, dark
nebulae are clouds of gas and dust which block the luminosity from any objects
behind them. The Horsehead Nebula, then, also in Orion, appears as it does because
dark material in the cloud completely blocks light from the stars beyond it.

Oops, it's time
to move on! We've seen young stars in the Orion Nebula, older ones in the Pleiades,
and still older ones in our own backyard (pssst -- the Sun is about five billion
years old). But what about the oldest stars we can see anywhere? To find those,
we'll need to head into the outer reaches of the Milky Way Galaxy -- into its
tenuous halo.

Home
to some of the oldest stars in the Universe, globular cluster NGC 6093
is striking in this Hubble Space Telescope image. Image courtesy of AURA/STScI
and NASA.

Up ahead is a globular
star cluster. Composed of a few thousand to a few million of the very oldest
stars in the Universe, globular clusters are beautiful markers to our galaxy's
limits.

Whew! That was
exhausting. But we have reached the edge of the Milky Way Galaxy. What lies
beyond, moving swiftly through the practical emptiness of the Universe, are
other galaxies -- some like ours, some not. The vast majority are, like the
Milky Way Galaxy and the Andromeda Galaxy (shown here), organized into clusters
of galaxies. And each galaxy, you must recall, is made up of star clusters,
nebulae, solitary stars, and planets.

You can spend
a lifetime studying our one planet and not see it all. Maybe a number of lifetimes
to study only a small portion of our Solar System. All these planets make up
just one solar system, around one star, in one galaxy. Our medium-sized galaxy
has a hundred billion stars, and we believe there are a few tens of billions
of galaxies in the observable Universe. Where does this continuum of enormity
end? We don't know, but we do know that there are countless voyages left to
take.

Kent Cheatham
is an avid amateur astronomer who lives just outside the lights of Harrah, Oklahoma.
Although in his job he controls the state's electrical grid for Oklahoma Gas
& Electric, his life centers around his wife and two children. His email address
is cheatham@ionet.net.